Towards an Urban Marine Ecology: Characterizing the Drivers, Patterns and Processes of Marine Ecosystems in Coastal Cities

doi: 10.1111/oik.05946 128 1215–1242

View metadata, citation and similar papers at core.ac.uk brought to you by CORE OIKOSprovided by Helsingin yliopiston digitaalinen arkisto Forum Towards an urban marine ecology: characterizing the drivers, patterns and processes of marine ecosystems in coastal cities

Peter A. Todd, Eliza C. Heery, Lynette H. L. Loke, Ruth H. Thurstan, D. Johan Kotze and Christopher Swan

P. A. Todd (https://orcid.org/0000-0001-5150-9323) ✉ ([email protected]), E. C. Heery and L. H. L. Loke, Experimental Marine Ecology Laboratory, Dept of Biological Sciences, National Univ. of Singapore, 16 Science Drive 4, Singapore 117558. – R. H. Thurstan, Centre for Ecology and Conservation, College of Life and Environmental Sciences, Univ. of Exeter, Penryn, UK. – D. J. Kotze, Faculty of Biological and Environmental Sciences, Ecosystems and Environment Research Programme, Univ. of Helsinki, Lahti, Finland. – C. Swan, Dept of Geography & Environmental Systems, Univ. of Maryland Baltimore County, Baltimore, MD, USA. Oikos Human population density within 100 km of the sea is approximately three times higher 128: 1215–1242, 2019 than the global average. People in this zone are concentrated in coastal cities that are hubs doi: 10.1111/oik.05946 for transport and trade – which transform the marine environment. Here, we review the impacts of three interacting drivers of marine urbanization (resource exploitation, pol- Subject Editor: Jarrett Byrnes lution pathways and ocean sprawl) and discuss key characteristics that are symptomatic Editor-in-Chief: Pedro Peres-Neto of urban marine ecosystems. Current evidence suggests these systems comprise spatially Accepted 3 May 2019 heterogeneous mosaics with respect to artificial structures, pollutants and community composition, while also undergoing biotic homogenization over time. Urban marine ecosystem dynamics are often influenced by several commonly observed patterns and processes, including the loss of foundation species, changes in biodiversity and productivity, and the establishment of ruderal species, synanthropes and novel assemblages. We discuss potential urban acclimatization and adaptation among marine taxa, interactive effects of climate change and marine urbanization, and ecological engineering strategies for enhancing urban marine ecosystems. By assimilating research findings across disparate disciplines, we aim to build the groundwork for urban marine ecology – a nascent field; we also discuss research challenges and future directions for this new field as it advances and matures. Ultimately, all sides of coastal city design: architecture, urban planning and civil and municipal engineering, will need to prioritize the marine environment if negative effects of urbanization are to be minimized. In particular, planning strategies that account for the interactive effects of urban drivers and accommodate complex system dynamics could enhance the ecological and human functions of future urban marine ecosystems.

Keywords: climate change, ecological engineering, ocean sprawl, pollution pathways, resource exploitation

Urban ecology has advanced rapidly in recent decades, yet has focused primarily on terrestrial and freshwater systems. By comparison, urban marine ecology is a field in its infancy and lacks the theoretical and empirical foundations underpinning urban ecosystem science on land. This Forum-article aims to help build such a foundation, by presenting a conceptual

Synthesis framework of the interacting drivers of marine urbanization, identifying key characteristics of urban marine ecosystems based on research from disparate disciplines, and highlighting research priorities that can advance urban marine ecology as a discipline.

–––––––––––––––––––––––––––––––––––––––– © 2019 The Authors. Oikos published by John Wiley & Sons Ltd on behalf of Nordic Society Oikos www.oikosjournal.org This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 1215 Introduction

The world’s population is urbanizing rapidly (Bloom 2011, Seto et al. 2011, UN 2017) with mass migration towards coastlines (Creel 2003, McGranahan et al. 2007) and policy reforms that favour densification (Dallimer et al. 2011, Kyttä et al. 2013). Population density at the coast (≤100 km from the sea and ≤100 m above sea level) is approximately three times higher than the global average and is increasing (Small and Nicholls 2003). Most people are concentrated in coastal cities that, as hubs for trade and/or due to a fertile delta, are frequently situated where river and sea meet (Konishi 2000). Many of these conurbations have expanded into megacities of more than ten million people (Nicholls 1995, Li 2003). For ecologists, coastal cities are of particular interest and concern, not only from a terrestrial perspective, but also in terms of consequences for, and interactions with, the marine environment (Dafforn et al. 2015, Firth et al. 2016). Understanding of the effects of urbanization on marine ecosystems and ecological processes is growing (Burt 2014, Mayer-Pinto et al. 2015, Firth et al. 2016). Human density is strongly related to resource exploitation, and one of Figure 1. Activities, installations, processes and issues that represent the early effects of marine urbanization is the depletion of instances of overlap and interaction among the three major drivers nearby fishery resources (Li 2003, Kirby 2004). Coastal cit- of marine urbanization: resources exploitation, ocean sprawl and pollution pathways. ies create marine pollution, including the harmful chemicals, bacteria and sediments associated sewage and urban runoff (Hoffman et al. 1983, Nixon 1995, Cornelissen et al. 2008). marine ecosystems are coupled social–ecological systems They also lead to nearshore development, usually starting and are heavily influenced by what is happening ‘upstream’ with a harbour, but also including hard coastal defences to in the urban fabric, by physical modifications nearshore and reduce erosion of valuable land, whether it be pre-existing or offshore, and by current and future consequences of climate reclaimed (Charlier et al. 2005, Lotze et al. 2005, Tian et al. change, such as sea-level rise and punctuated extreme weather 2016). These artificial structures have significant effects on events. As such, the dynamics and prevailing ecological para- the ecology of shorelines, especially when entire habitats are digms for these systems have yet to be tested experimentally, replaced with novel materials such as concrete and gran- and it is only through expanded field manipulations that it ite (Firth et al. 2014, Dyson and Yocom 2015, Loke et al. will be possible to understand the core properties of urban 2019a). marine ecosystems: how they are structured, how they func- Several recent reviews have separately highlighted urban- tion and the key parameters that drive the ecosystem services related pollution and physical modifications of urban shore- they provide. lines as critical components of urban marine ecosystem In this paper, we outline the primary drivers of marine dynamics (Dafforn et al. 2015, Firth et al. 2016, Heery et al. urbanization and identify the known patterns exhibited by 2018a), but exploitation of marine resources is rarely dis- marine ecosystems in urban areas. Empirical testing of the cussed in an urban context (though see Li 2003, Baum et al. underlying processes that create these patterns and further 2016). The overarching characteristics of urban marine eco- research in areas we highlight in this paper can help build a systems that result from each of these factors and their poten- framework for understanding multifaceted impacts of marine tial combined effects have yet to be thoroughly considered. urbanization, and future trajectories of urban marine ecosys- There is considerable need to integrate findings relating to tems in the face of climate change. marine urbanization across subdisciplines of ecology; this effort would be aided by conceptual frameworks that integrate multiple variables, identify potential interactions and Three main drivers of marine urbanization feedbacks, incorporate historical trajectories, and facilitate the development of testable hypotheses regarding the response of The process of marine urbanization comprises three primary urban marine ecosystems to further environmental change. drivers (Fig. 1). The first is exploitation of both living and Frameworks meeting this need would not only broadly non-living resources (see ‘Resource exploitation (both liv- support marine research in the Anthropocene, as nearly all ing and non-living)’) and includes recreational, subsistence coastal zones are now strongly impacted by anthropogenic and commercial fishing, as well as dredging and mining for stressors, but would also help build a foundation for urban minerals (Table 1). In post-industrialized nations, this may marine ecology – a field in its nascence. Inevitably, urban largely be historical, but with long lasting effects that are still

1216 Table 1. Types of marine exploitation and their scope, scale and potential effects.

Exploitation type Scope and scale Potential effects on marine life and habitats Recreational fishing Estuarine, inshore, offshore; Removal of target fish and shellfish, potentially leading to population- scale can range from hundreds and/or ecosystem-wide impacts. Delayed mortality from catch and to tens of thousands of release practices; mortality of bycatch species; mortality or injury participants in a region. from boat collisions. Damage or degradation of sensitive habitats from contact fishing gear or the launching/recovery of boats. Lost and abandoned fishing gear issues (Table 2). Subsistence fishing and Estuarine, intertidal, inshore; Removal of target fish and shellfish, potentially leading to population- gleaning numbers unknown but likely and/or ecosystem-wide impacts; mortality of bycatch species; to vary greatly by region. mortality or injury from boat collisions. Damage or degradation of sensitive habitats from contact fishing gear and the launching/ recovery of boats. Impacts from practices such as cyanide or dynamite fishing. Lost and abandoned fishing gear issues (Table 2). Commercial fisheries Estuarine, inshore, offshore; Removal of large numbers of target species, potentially leading to major scale variable by fishery and population- and/or ecosystem-wide impacts; mortality of bycatch region but can range from species; mortality or injury from boat collisions; damage or degradation tens to thousands of of sensitive habitats from contact fishing gear and the launching/ participants. recovery of boats. Lost and abandoned fishing gear issues (Table 2). Mariculture Estuarine, inshore (offshore Transmission of disease and parasites between farmed and native in the future); scale varies species; eutrophication due to addition of nutrients (although widely and depends upon the shellfish farms may remove nutrients from water column); smothering species being farmed. of benthic fauna due to build-up of organic material (also leading to changes to sediment type/chemistry). Dredging for minerals/ Inshore and offshore; scale variable Physical disturbance and removal of the substrate and associated aggregates but can range benthic biota; changes to the composition of the sediment/substrate; from tens to hundreds of km2. changing bathymetry and sediment transport patterns; smothering of biota; reduced light and enhanced turbidity due to sediment suspension, toxicant release (Table 2). Beach mining Inshore; usually conducted at the Direct removal of species and substrate; loss of soft-sediment habitat; local scale but with possible lowering/loss of beach leading to erosion, changing sediment regional-scale effects. transport patterns, increased turbidity, changing conditions for fauna/ flora and/or saline water intrusion. Oil and gas extraction Inshore and offshore (mostly Direct removal of species and substrate; smothering/physical alterations offshore in recent years); to habitat/substrate type (i.e. replacement of soft with hard substrate); local to regional-scale effects. chronic and acute toxic pollution events; noise pollution (Table 2). Water extraction for Inshore, generally Fish and plankton killed during intake and processing (impingement cooling and localized effects. and entrainment). Brine and heated water (thermal pollution) desalination can impact communities near outflows, changing behavior and physiology. Toxicants can also be released with the effluent. relevant today. The second is pollution (see ‘Pollution path- these primarily as they relate to one or more of the three driv- ways (both industrial and domestic)’), including sediments, ers presented below. industrial and municipal waste, domestic wastewater a, animal/slaughterhouse waste, fecal matter, street dust, oil from Resource exploitation (both living and non-living) automobiles and other contaminant sources, pharmaceuticals, light pollution, and noise pollution (Table 2). The third It is increasingly well documented that the overexploitation is the wholesale conversion of natural habitats into a different of living coastal and marine resources is one of the earliest state (see ‘Ocean sprawl (both coastal and offshore)’), such observable forms of human disturbance within coastal eco- as reclaimed land, seawalls, jetties, piers, marinas, groynes, systems (Jackson et al. 2001, Pandolfi et al. 2003, Lotze et al. breakwaters, port and harbor infrastructure, and bridges 2006). Moreover, coastal systems that have endured the lon- (collectively termed as ‘ocean sprawl’, Table 3). These three gest period of intense human impacts and that contain the drivers are presented in the chronological order in which highest human populations are among the most degraded they often begin to occur, though their timing and relative (Lotze et al. 2006). Yet, awareness of the magnitude of changes scope can vary substantially among cities (Fig. 2). Further, that previously occurred as a result of the exploitation of liv- the three drivers can have interactive effects, with potential ing and non-living marine resources is generally poor. This additional consequences for marine ecosystems (see ‘Overlap, is due to exploitation usually commencing prior to regular interactions and feedbacks’). Other factors relating to urban- monitoring of these systems, coupled with the pervasiveness ization, such as elevated propagule pressure and invasion of the shifting baseline syndrome, where a lack of knowledge risk, can also be particularly intense in coastal cities (Carlton of past ecological conditions facilitates a gradual ratcheting 1996, Ruiz et al. 1999, 2000, Mineur et al. 2012, but see down of expectations as to what constitutes a healthy ecosys- Tan et al. 2018 and Wells et al. 2019), however, we discuss tem (Pauly 1995, Sheppard 1995).

1217 Table 2. Pathways and potential effects of pollution on marine life.

Pollutant type Main urban pathways Potential effects on marine life Sediments Construction sites (on the coast Turbidity resulting in less light for photosynthesis and visual and within inland urban areas), predators/prey. Down welling sediments smother benthic dredging, land reclamation. organisms and create a substrate unsuitable for settling larvae. Nitrogen and phosphorus Industrial discharge, human and Eutrophication leading to both micro and macro algal blooms, animal waste, detergents, reduced water clarity (see ‘sediments’), shifts toward noxious mariculture. cyanobacteria and reduced fertilization success in corals. Plastics (macro and micro), Resin pellets and discarded Ingestion and/or entanglement, leading to internal blockages/ lost and abandoned end-user products. Fishing injuries, toxic poisoning, starvation due to false ‘stomach filling’, fishing gear activities. suffocation, lacerations, infections, reduced ability to swim. Compounds from oil Motor vehicles, shipping, Impairment of growth and developmental rates, reduced industry. reproductive output and recruitment rates, increased susceptibility to disease. Carcinogenic. Heavy/trace metals Industrial and vehicle emissions, Can inhibit fertilization, recruitment, development, growth in leaching from landfills, urban marine microorganisms, invertebrates and vertebrates. runoff, sewage. Carcinogenic. Prone to undergo food chain magnification. Tributyltin Antifouling paint used in the Causes imposex, and reduces growth and larval success, in maritime industry. various crustaceans and molluscs. Biomagnifies, leading to endocrine disruption in fishes, marine mammals and humans. PCBs and PBDEs Discharge from industry, especially Prone to biomaginification. Interferes with neurological and electronics. Used in plastics, hormonal systems of marine organisms and humans. Can lead fire retardants and lubricants. to decreases in reproductive capabilities and pose immunotoxic risk in marine mammals. Pharmaceuticals Industrial, hospital and domestic Interferes with reproduction and development in both animals and waste. plants. Perturbs fish physiology. Bacteria and viruses Sewage (from land and boats/ships), Diseases, especially acute gastrointestinal illnesses, e.g. aquaculture. salmonellosis. Viruses can cause hepatitis and respiratory infections. Light Streets, private and commercial Encourages unwanted fouling, affects migration and predator–prey buildings, vehicle headlights, behavior. Disrupts larval settlement. De-synchronization of airports. broadcast spawning from lunar phase (e.g. corals). Noise Boat traffic, construction, Disrupts behavior (e.g. ability to find food, mates or avoid machine operation. predators), reduces growth and fecundity.

Coastal population growth and development has Overexploitation often follows a predictable spatio–tem- impacted a wide variety of living marine resources (Table 1). poral pattern that is tied to urban growth. This is particularly For instance, oyster reefs and maerl beds have dramatically evident among exploited sessile species. On the east coast of declined or been extirpated in coastal ecosystems around the the United States, historical oyster fishery collapses demon- world due to destructive fishing methods aimed at provid- strated sequential depletion beginning in urbanized estuar- ing food and/or building material for increasingly urbanized ies and spreading along the coast away from urban centres populations (Airoldi and Beck 2007, Claudet and Fraschetti (Kirby 2004). Many European native oyster reefs adjacent 2010). Human population growth facilitated the establish- to urban conurbations became ecologically extinct prior to ment and expansion of industrialized commercial harvesting the mid-20th century (Korringa 1946, Airoldi and Beck for marine mammals, turtles and fin-fish species, ultimately 2007, Thurstan et al. 2013). OysterOstrea angasi reefs in resulting in the decline or loss of marine megafauna, and of South Australia disappeared less than 200 years after the first diadromous and large demersal fish species (Lotze et al. 2005, records of commercial oyster landings from this region by Thurstan et al. 2010, Van Houtan and Kittinger 2014). early Europeans (Alleway and Connell 2015). A total of five Targeted fin-fish assemblages, although constrained by envi- species of giant clam were historically recorded in the coastal ronmental factors (e.g. availability of suitable habitat), have seas around Singapore, but now only two remain, and these been shown to decline in abundance and richness along only exist in very low abundances (Neo and Todd 2012). increasing gradients of human pressure or proximity to urban The intensification of giant clam exploitation in the 19th centres in a range of habitats (e.g. coral reefs: Williams et al. century, followed by extensive coastal development from the 2008, Brewer et al. 2009, Aswani and Sabetian 2010; surf 1960s onwards, are considered to be the main drivers in the zones of exposed sandy beaches: Vargas-Fonseca et al. 2016). decline and extirpation of these charismatic invertebrates Fishing effort also impacts intertidal species abundance, for (Guest et al. 2008, Neo and Todd 2012). example, the majority of known sandy beach invertebrate The historical legacy effects of overexploitation, combined fishery stocks are fully exploited, overexploited or depleted with pollution and coastal development, means that the pres- due to commercial, subsistence or recreational harvesting ent day commercial exploitation of living marine resources (Defeo and de Alava 1995, Defeo 2003). adjacent to urbanized regions, at least in more economically

1218 Table 3. Types of human-made structures comprising ocean-sprawl, their functions and potential impacts. Note: All of these structures require some alteration and/or loss of natural habitat.

Structure type Function Potential effects on marine habitats Reclaimed land and Alleviation of coastal squeeze and Directly results in habitat loss, and fragmentation. Sedimentation artificial islands expansion of land for industry and during construction, altered hydrodynamics interferes with development. connectivity at landscape and local scales. Artificial coastal Engineered to protect shorelines from Reduced intertidal extent resulting in steeper slopes. Footprint of the defenses shoreline erosion, flooding and impacts structure removes existing natural habitat but effects may extend from waves. beyond structure (halo effect). Change in substrate material and altered hydrodynamics could result in different colonizing assemblages. Commercial ports, Industry, services and recreation. Elevated risk of species invasions, contaminants (oil, antifouling docks and marinas coatings, noise, light), disturbances associated with shipping (sediment resuspension, propeller injuries, etc.). Oil shipping and Non-renewable resource mining for Footprint of the structure removes existing natural habitat but effects refinery infrastructure energy. may extend beyond structure (halo effect). Contaminants, risk of oil spills, noise and light pollution. Tidal and wind energy Energy production. Footprint of the structure removes existing natural habitat but effects infrastructure may extend beyond structure (halo effect). Noise and light pollution, electromagnetic fields. Submarine cables and Telecommunications, power, water, oil. Concrete mattresses are often used to stabilize and position cables pipelines on seafloor. Fragmentation of soft-sediment habitats due to introduction of hard substrates. Noise and light pollution during construction phase. Electromagnetic fields. developed countries (MEDCs), is often far lower than its his- communities in or near urban areas (Smit et al. 2017). The torical peak (Lotze et al. 2005, 2006). The search for resources maintenance of these traditional activities is, however, under has thus moved further offshore and into less exploited regions pressure from factors such as declining water quality and (Swartz et al. 2010, Anderson et al. 2011). Recreational fish- coastal development (Smit et al. 2017), as well as enhanced ing participation rates in MEDCs have also seen a decline in access to education and alternative employment opportuni- the last two decades as a result of factors related to urbaniza- ties for children of fishing families (Teixeira et al. 2016). In tion, such as increased urban sprawl, demographic change, some cases, urbanization may enhance economic opportuni- and a reduction in fishable water resources (Poudyal et al. ties for small-scale fishing communities. In southern Brazil, 2011). In contrast, within less economically developed for example, the proximity of small-scale fishers to urban countries (LEDCs), small-scale and subsistence fishing centres has expanded opportunities for subsistence fishers to often remains a significant source of livelihood for coastal access additional markets, as the presence of high numbers

Figure 2. Trajectories of the three key drivers of marine urbanization over time are difficult to hindcast (or forecast) and are likely to be city- specific. However, they will almost certainly overlap, potentially creating non-linear interactions that are even more challenging to predict (and are not represented here). For illustration purposes only: (a) the exploitation of living resources could accelerate rapidly during the early development of many coastal cities, yet decrease in intensity as the resource is overexploited or inaccessible due to other factors, such as contaminants. Conversely, ocean sprawl may be more likely to follow an asymptotic trajectory, which reaches saturation as an increasingly large percentage of natural habitats are converted by the installation of artificial structures. (b) A possible alternative configuration of driver trajectories in a younger city with a shorter but equally intense history of marine urbanization.

1219 of fishers enables them to supply enough fish to meet supply Pollution pathways (both industrial and domestic) chain demand (Hellebrandt 2008). Urbanization also coincides with increases in the exploi- Urbanization and pollution are tightly linked; whereas as air tation of non-living resources, including the extraction of and soil pollution are major concerns for terrestrial conur- marine aggregates (sand, gravel, rocks) for use in construc- bations, contaminated water and sediments are additional tion and beach renourishment, mineral resources for indus- and often critical pollution issues for coastal cities (Table 2). trial applications, and the extraction of energy resources (oil Originating from both point (e.g. wastewater discharge) and and natural gas, and wave and tidal resources). Nearshore non-point (e.g. wind-blown debris and dust) sources, pol- aggregate dredging may occur for mud, rock, shells, corals or lution impacts marine life at individual, population and sand for construction purposes, or for the heavy or precious ecosystem levels, frequently bioaccumulating and then bio- minerals they contain (Charlier and Charlier 1992). Potential magnifying up the trophic pyramid (Erftemeijer et al. 2012, negative effects arising from the extraction of coastal marine Johnston et al. 2015, Langston 2017). Chronic marine pol- aggregates include an increased risk of flood events and coastal lution effects tend to be sub-lethal (Browne et al. 2015), but erosion. For example, aggregate extraction from the coasts of they can interact with other stressors in ways that ultimately Kiribati in the South Pacific resulted in beach structure being cause mortality (Yaakub et al. 2014a, Bårdsen et al. 2018). degraded, exposing coastal conurbations to enhanced risk Urban sediment pollution, commonly the result of runoff of flood events (Webb 2005, Holland and Woodruff 2006). from construction work and disturbance via dredging (Rogers Similarly, beach mining, nearshore dredging and quarry- 1990, Eggleton and Thomas 2004, Erftemeijer et al. 2012), ing have contributed significantly to coastal erosion in the as well as other sources such as beach nourishment and land- Marshall Islands (Holland and Woodruff 2006), France, and use changes that alter catchment runoff (Colosio et al. 2007, Bali (Charlier and Charlier 1992). The extraction of sand for Zhang et al. 2010), affects marine life in multiple ways. The the renourishment of urban beaches is commonly undertaken resulting increase in turbidity reduces light penetration, pho- for aesthetic and erosion control purposes (Fletemeyer et al. tosynthesis (Falkowski et al. 1990), and the maximum depth 2018). Knowledge of the direct and indirect effects of this at which photosynthetic organisms can grow (Heery et al. activity on the local biota and ecological processes remains 2018a). Suspended sediments also reduce fish hatching suc- incomplete (Peterson and Bishop 2005), but beach renour- cess and larval survival (Auld and Schubel 1978), impede ishment has been shown to negatively impact nearshore zooplankton feeding (Sew et al. 2018), affect mobile fauna coral reefs (Hernández-Delgado and Rosado-Matías 2017), that rely on visual cues (Weiffen et al. 2006), and alter a wide marine invertebrate prey availability and nesting behavior in range of benthic ecosystem processes and patterns (Airoldi sea turtles (Peterson and Bishop 2005). Coastal urbanization 2003), including the settlement and successful recruitment of also facilitates the expansion of maritime port operations, organisms, the diversity of species, and competitive interac- which often dredge nearshore channels to maintain deep- tions – such as those between foundation macrophyte species water access for commodity and passenger transport (Lemay and low-lying algal turfs (Gorgula and Connell 2004, Russell 1998). Dredging and mining represent a major area of over- and Connell 2005, Gorman and Connell 2009, Knott et al. lap between exploitation and pollution (Fig. 1) due to the 2009, Bauman et al. 2015). Smothering by sediment further release of toxicants and sediments that occurs during these reduces light and physically interferes with the functioning operations. of benthic organisms like corals (Rogers 1990, Junjie et al. The establishment of oil and natural gas rigs can be bro- 2014), seagrasses (Erftemeijer and Lewis 2006), and certain ken down into four stages: seismic exploration, exploratory life stages of kelps (Devinny and Volse 1978, Geange et al. drilling and installation, operation and decommissioning 2014). (Khan and Islam 2008). Each of these stages involves some High nutrient concentrations are frequently attendant form of extractive activity, although the consequences for with sediments but, in urban settings, inputs come also from marine life are particularly strong during the installation and wastewater treatment plants, industrial discharges, storm- decommissioning stages. The installation and decommission water runoff, dust from land, domestic detergent use and of rig infrastructure may also degrade or destroy the seabed human sewage (McClelland et al. 1997, Braga et al. 2000, (Macreadie et al. 2011). However, their establishment intro- Atkinson et al. 2003, Cole et al. 2004, Gaw et al. 2014, Vikas duces a source of hard substrate, potentially increasing local and Dwarakish 2015) and are particularly hazardous in bays biodiversity, as well as non-native species, which can alter and harbors with limited circulation (Gomez et al. 1990). community dynamics at local or regional levels (Burt et al. Resultant eutrophication can have positive feedbacks on nutri- 2009, Feary et al. 2011, Macreadie et al. 2011). The estab- ent loads and localized acidification (Howarth et al. 2011) lishment of renewable energy infrastructure presents many of and leads to many undesirable ecological effects (Bell 1991, the same ecological issues and opportunities as oil and gas, yet Orth et al. 2017), for example phytoplankton blooms and/ the installation of some structures, such as tidal barrages, has or shifts toward noxious cyanobacteria, macroalgal blooms the potential for generating significant physical and ecologi- that can outcompete foundation species such as corals, and cal impacts at the local scale, including the loss of intertidal increases in the occurrence and severity of marine diseases habitats, modification of water flow and sediment resuspen- (Bowen and Valiela 2001, Balestri et al. 2004, Lapointe et al. sion (Gao et al. 2013, Hooper and Austen 2013). 2005, Reopanichkul et al. 2009, Haapkylä et al. 2011,

1220 Redding et al. 2013). Human sewage and wastewater cre- Artificial lighting has also been reproted to affect predator ates additional problems due to the release of fecal coliforms, and prey behavior, disrupt larvae settlement, alter distribu- antibiotics and other pharmaceuticals (Jiang et al. 2001a, tion patterns and de-synchronize broadcast spawning species Shibata et al. 2004, Watkinson et al. 2007, Rose et al. 2009, from normal lunar phases (Becker et al. 2013, de Soto et al. Jia et al. 2011, Rizzo et al. 2013, Gaw et al. 2014). 2013, Wale et al. 2013, Navarro-Barranco and Hughes 2015, Toxic pollutants, including organochlorine compounds Bolton et al. 2017). (e.g. PCBs and HCH), heavy metals, tributyltin (TBT), A gradient of decreasing levels of various pollutants polybrominated diphenyl ethers (PBDEs) and compounds with increasing distance from urban sources has been from oil (e.g. petrogenic PAHs, plastics and microplastics), described multiple times, particularly for: heavy metals are strongly associated with industrial activities and urban (Qiao et al. 2013), sediments (Todd et al. 2004), marine run-off (Kennish 1997, Shazili et al. 2006, Todd et al. 2010, debris (Evans et al. 1995, Andrades et al. 2016), and PAHs Cole et al. 2011, Tayeb et al. 2015), as well as from ship- (Assunção et al. 2017). Whereas the effects of urban (land- ping and other sea-based sources (Tornero and Hanke 2016). based) light and noise pollution and some contaminants are Many of these substances bioaccumulate in animals (Tanabe limited to a few decimeters to kilometers from the source 1988, Wolff et al. 1993, Bayen et al. 2003) interfering with (Zaghden et al. 2005, Burton and Johnston 2010), other pol- cellular and biochemical functions and disrupting hormonal, lutants have impacts that extend much further (Heery et al. reproductive, neurological and nervous systems (Portmann 2017). For example, PCBs have been found in Arctic waters 1975, Wolff et al. 1993, Frigo et al. 2002, Bosch et al. 2016). far from any urban or industrial centres, albeit at very low Lead, cadmium, copper, tin, nickel and iron are among the levels (Gioia et al. 2008). An important example of urban metals commonly found in sediments near industrial areas pollution being transported huge distances but still having a (Williamson and Morrisey 2000, Buggy and Tobin 2008, substantial negative impact is marine debris, especially plas- Amin et al. 2009). Copper is especially toxic to marine tics. Like other forms of marine debris, plastics have a very invertebrates, including poriferans, cnidarians, molluscs and high dispersal potential (Carlton et al. 2017), mainly because arthropods (Reichelt-Brushett and Harrison 1999, 2000, they can take decades to biodegrade (Moore 2008) and are Johnston and Keough 2000, Brown et al. 2004, Rainbow often buoyant. They can maintain their structural integrity 2017). The impacts of lead and cadmium on economically for many years, resulting in negative effects, via ingestion or important invertebrates such as oysters and crabs are also entanglement, to animals ranging from seabirds, turtles and well established in the literature (Ramachandran et al. 1997), marine mammals to crustaceans and cnidrians (Azzarello and however, recent studies suggest deleterious effects from a Van Vleet 1987, Moser and Lee 1992, Bjorndal et al. 1994, wide range of metals (Langston 2017), particularly when Jones 1995, Laist 1997, Lamb et al. 2018, Mecali et al. 2018) combined with other anthropogenic stressors (Burton and far from their point of origin. Due to ultraviolet rays, mechan- Johnston 2010). Other industrial discharges that are known ical and microbial degradation, plastics eventually fragment to have negative effects, albeit usually localized, include brine into microplastics (Thompson et al. 2004, Barnes et al. 2009) that are bioavailable to suspension feeding marine organisms, from desalination plants and heat from industrial cooling. including zooplankton (Browne et al. 2008, Wright et al. Often the most deleterious impacts from these discharges are 2013, Barboza et al. 2018, Botterell et al. 2018). toxicants (especially metals, hydrocarbons and anti-fouling compounds) that enter the sea with the effluent (Lattemann Ocean sprawl (both coastal and offshore) and Höpner 2008, Roberts et al. 2010). Urban noise pollution usually originates from boat traf- ‘Ocean sprawl’ is a term used to describe the proliferation fic and in-water construction (Middel and Verones 2017) of human-made hard structures in the marine environment while urban light pollution comes from street lights, build- (Duarte et al. 2013, Firth et al. 2016, Table 3). This encom- ings, shipping, airports and vehicle headlights (Hölker et al. passes offshore infrastructure (e.g. wind farms, oil and gas 2010). For some fish and marine mammals, noise pollu- platforms, aquaculture facilities, submarine cables/pipes) and tion inhibits communication, affects predator–prey interac- coastal infrastructure such as artificial shore defences (e.g. sea- tions, and has negative effects on growth and reproduction walls, breakwaters, groynes), as well as facilities associated with (Slabbekoorn et al. 2010, Houghton et al. 2015). It may ports, docks and marinas. Ocean sprawl is a fundamental and also impact various other taxa that are sensitive to sound, dominant feature of urbanized marine environments (Bulleri such as oysters (Charifi et al. 2017), clams (Mosher 1972, and Chapman 2010, Duarte et al. 2013, Dafforn et al. 2015, Peng et al. 2016), mussels (Roberts et al. 2015), cepha- Firth et al. 2016) with artificial structures comprising the lopods (André et al. 2011, Fewtrell and McCauley 2012), bulk of shorelines in many coastal cities (Bulleri et al. 2005, shrimp and other invertebrates (Solan et al. 2016). Night Todd and Chou 2005, Dafforn et al. 2015, Lai et al. 2015) lighting comprises both direct glare and overall increased and modifying habitats well into the subtidal zone (Airoldi illumination, and can disrupt marine ecosystems in a num- and Beck 2007, Heery et al. 2017, Heery and Sebens 2018, ber of ways (Hölker et al. 2010). Organisms that use light Macura et al. 2019). to navigate, such as birds and sea turtles, may become dis- As a habitat, artificial shorelines are quite distinct orientated (Davies et al. 2014), as may fish and fish larvae. from natural rocky shores (Connell and Glasby 1999,

1221 Rilov and Benayahu 2000, Perkol-Finkel and Benayahu development (Jiang et al. 2001b, Kennish 2002, Finkl and 2004, Bulleri et al. 2005, Moschella et al. 2005, Clynick et al. Charlier 2003, Mayer-Pinto et al. 2015). This overlap can 2008, Lam et al. 2009, Bulleri and Chapman 2010, Lai et al. have important consequences for marine organisms and 2018). One of the most obvious differences is the slope of communities, as effects from multiple anthropogenic stress- hard substrates; while shoreline armoring structures such as ors are often cumulative and non-linear in the marine envi- seawalls are generally very steep, natural rocky shores tend to ronment (Adams 2005, Crain et al. 2008, 2009), leading be more gently sloping with longer and wider intertidal areas to complex changes in ecosystem condition (Conversi et al. (Gabriele et al. 1999, Knott et al. 2004, Andersson et al. 2015, Halpern et al. 2015, Möllmann et al. 2015). It can 2009, Chapman and Underwood 2011, Firth et al. 2015). also feedback to influence the key drivers themselves, which The smaller area of intertidal zone typical of seawalls is prob- are each the result of dynamic, interacting socio-economic ably an important contributor to species loss (Chapman and biophysical forces (sensu Alberti et al. 2003), and closely and Underwood 2011, Perkins et al. 2015) as it can lead to interrelated in the coupled social-ecological systems that greater overlap in the distribution of individuals (Klein et al. characterize coastal cities (Liu et al. 2007, Alberti 2008, 2011) or to superimposed distributions of species that would Grimm et al. 2008a, Pickett et al. 2011). Such feedbacks and not normally occur (Lam et al. 2009, Loke et al. 2019b). interactions are widely recognized as shaping urban ecosystem Wave impact is also more intense on steep shores (Gaylord function (Wu 2014, McPhearson et al. 2016), and are cen- 1999, Cuomo et al. 2010), potentially dislodging intertidal tral in nearly all current models of urban ecosystem dynamics organisms and/or impeding their settlement (Blockley and (Pickett et al. 2001, Alberti et al. 2003, Grimm et al. 2013). Chapman 2008, Iveša et al. 2010). Compared to natural In this section, we highlight some known and likely inter- hard-bottom habitats, seawalls are topographically ‘sim- actions among the three drivers (exploitation, pollution and ple’ (Loke et al. 2014) – having few microhabitats, such as ocean sprawl) of marine urbanization. Each interaction fits pits, rock-pools, overhangs and crevices (Chapman 2003, conceptually within the overlapping regions of the Venn dia- Chapman and Bulleri 2003, Moreira et al. 2007), which are gram in Fig. 1. important for the persistence of many intertidal and benthic One of the best examples of complex interactions and species (Chapman and Underwood 2011, Loke and Todd feedbacks among the drivers of marine urbanization and 2016, Loke et al. 2017). When considering these multiple ecosystems is the relationship between habitat conversion, effects in combination, it is unsurprising that many direct contaminants and invasion risk. Artificial structures associ- comparisons between rocky shores and seawalls often reveal ated with port infrastructure and shoreline protection tend the latter host lower species richness, reduced functional and to both concentrate environmental contaminants by alter- genetic diversity, and different community compositions ing hydrodynamic patterns and reducing water movement (Chapman 2003, Bulleri et al. 2005, Moschella et al. 2005, (Waltham et al. 2011, Rivero et al. 2013), and by facilitat- Fauvelot et al. 2009, Lai et al. 2018). ing increased contaminant influx, for instance from antifoul- The consequences of ocean sprawl at large spatial scales ing paints (Schiff et al. 2004, 2007, Warnken et al. 2004, are not yet well understood, but they are likely to be con- Sim et al. 2015). Copper emissions from antifouling paints siderable given its prominence and extent (Lotze et al. 2006, then have both direct and indirect consequences for marine Airoldi and Beck 2007). In some heavily urbanized regions, organisms (Rygg 1985, Perrett et al. 2006). The toxin enters entire habitats have been lost as artificial structures prolifer- the food web by accumulating in algal tissues (Johnston et al. ate over vast distances (Dong et al. 2016). Even where coastal 2011) or being consumed directly by non-selectively feed- transformation is not ubiquitous, clusters of artificial struc- ing animals, which can additionally accelerate the leaching tures can serve as corridors that facilitate species invasions (Airoldi et al. 2015) and alter ecological connectivity, with and burial process in adjacent sediments (Turner 2010). significant effects on marine assemblages (Bishop et al. 2017). While toxic effects from copper negatively impact many The spatial scale of impacts from artificial structures depends marine organisms and reduce diversity (Rygg 1985), differ- on the type of structure, local hydrodynamic conditions, and ential responses to copper contamination among inverte- a variety of other parameters (summarized by Heery et al. brates (Piola and Johnston 2006) combined with the novel 2017). For instance, fluxes of exogenous detritus from arti- colonization habitat that is provided by floating docks and ficial structures typically affect marine communities within other marina structures can disproportionately favor non- meters to tens of meters only (Heery and Sebens 2018), while indigenous taxa, thus facilitating marine invasions (Piola infrastructure that creates major impediments to circulation and Johnston 2008, Dafforn et al. 2009, Piola et al. 2009, and sediment transport tends to impact marine assemblages Airoldi and Bulleri 2011, Edwards and Stachowicz 2011, across a much larger area (Bishop et al. 2017). Cordell et al. 2013, McKenzie et al. 2012). The trajectory of marine resource exploitation in urban Overlap, interactions and feedbacks areas is also closely tied to that of pollution pathways and ocean sprawl (Inglis and Kross 2000, Jiang et al. 2001b, The three key drivers described above are not limited to Cundy et al. 2003) (Fig. 2). In the early developmental stages urban areas, yet their relative magnitude and spatial and tem- of many cities, shoreline habitats were converted by artificial poral overlap is often augmented near high-density coastal structures to facilitate resource exploitation industries and

1222 the economic growth they fueled (Squires 1992). Overwater ecosystem characteristics and several key ecological patterns, structures that housed cannery facilities and seafood markets which are just beginning to emerge in the literature. were prominent drivers of early waterfronts in San Francisco (Walker 2001), Singapore (Chang and Huang 2011), and Homogenized systems, comprising heterogeneous many other coastal cities globally (West 1989, Portman et al. mosaics 2011). Various shoreline armoring structures were also part of facilities for resource exploitation industries, such as oil A common theme in the terrestrial urban ecology literature and gas (Minca 1995), and remain important drivers in adap- is the spatial heterogeneity that occurs across landscapes as tation plans for protecting these industries from future sea a result of urbanization (Pickett et al. 1997, Dow 2000, level rise (French et al. 1995, Ng and Mendelsohn 2005). Cadenasso et al. 2007, Pickett and Cadenasso 2008). The Pollution associated with resource exploitation and habi- resulting ‘mosaics’ of habitat types, biophysical charac- tat conversion continues to be problematic in many urban teristics, and land use are temporally dynamic and influ- and suburban areas, for instance surrounding shellfish enced by multiple interacting social and ecological drivers aquaculture farms, oil refineries, port infrastructure and (Pickett et al. 2017). At the same time, there are consider- dredged waterways that harbor contaminants (Board 1997, able similarities across cities in the underlying processes and Pereira et al. 1999, Jones et al. 2001, Strand and Asmund trajectory of urbanization, leading to an overall homogeniza- 2003, Tolosa et al. 2004, Medeiros et al. 2005, Casado- tion among urban ecosystems regionally and globally (Alberti Martínez et al. 2006, Paissé et al. 2008, Knott et al. 2009), 2005, McKinney 2006). Even though research supporting and alters system dynamics via multiple biogeophysical path- these concepts is far more comprehensive in terrestrial envi- ways, trophic levels and functional groups (Paissé et al. 2008, ronments, there are several indications of comparable pat- Weis et al. 2017). terns among urban marine ecosystems based on the current As coastal cities grow, and effects from various aspects of literature (Dafforn et al. 2015). marine urbanization increasingly overlap (Fig. 2), the system’s Most coastal cities are positioned in estuaries and bays that potential for feedbacks appears to intensify (Fernando 2008, were historically dominated by soft sediments. As artificial Grimm et al. 2008b). For instance, as impervious surfaces structures are added to these sedimentary environments, a proliferate on land, increased delivery of stormwater can checkerboard of hard and soft habitats is created, with each accelerate the accumulation of contaminants in receiving supporting distinct biotic assemblages (Connell and Glasby waterbodies (Lee et al. 2006, Jartun et al. 2008, 2009, Jartun 1999, Glasby 2000, Connell 2001, Barros et al. 2001). and Pettersen 2010, Walsh et al. 2012). Similarly, as resource This can alter ecosystem dynamics in several ways. In some exploitation and shoreline alteration expand, so too does the regions, artificial structures support a larger standing stock spatial extent and magnitude of marine debris and contami- of benthic macroalgae and other hard-bottom organisms, nants (Garcia-Sanda et al. 2003, Wake 2005, Ng and Song which then enter adjacent sediments as detritus and may 2010, Märkl et al. 2017), which can in turn impact exploit- alter sedimentary community dynamics (Boehlert and Gill able marine resources (Islam and Tanaka 2004). Additional 2010, Heery 2018, Heery and Sebens 2018). Artificial struc- biogeochemical and ecological feedbacks have also been tures has been shown to o act as ‘stepping stones’ for disper- important historically, in some cases leading to losses in a sal, particularly of non-indigenous taxa (Bulleri and Airoldi system’s capacity to absorb urban impacts over time (Cloern 2005, Glasby et al. 2007, Vaselli et al. 2008, Sheehy and Vik 2001, Nyström et al. 2012). For instance, the loss of oyster 2010, Airoldi et al. 2015, Foster et al. 2016) and alter genetic reefs due to overharvesting and eutrophication is thought to population structure of marine fauna (Fauvelot et al. 2012). have reduced the filtration capacity of urban estuaries in the Marine species vary in dispersal potential, and many taxa United States (Zimmerman and Canuel 2000, Kemp et al. encounter barriers to dispersal at relatively small spatial scales 2005, Wilberg et al. 2011, zu Ermgassen et al. 2013), poten- (Darling et al. 2009, Costantini et al. 2013, Maas et al. 2018, tially inhibiting their ability to accommodate further pollu- Sefbom et al. 2018). Dispersal limitation can therefore also tion influx. Similar feedbacks surrounding challenges such as interact with local stressors and abiotic conditions to result in harmful algal blooms and marine diseases may be increas- compositionally very different assemblages across patches of ingly likely as ecosystems are further altered by marine urban- hard substrata (Bulleri and Chapman 2004, Munari 2013). ization (Prins et al. 1997, Sunda et al. 2006, Heisler et al. This may be accentuated where urban habitat conversion has 2008, Crain et al. 2009). However, such feedbacks can be significantly altered hydrodynamic patterns, created other difficult to predict and may obfuscate efforts to effectively additional barriers to dispersal and subsequent settlement anticipate ecosystem response to further environmental (Bishop et al. 2017) or changed the configuration of habitats change (Elmqvist et al. 2003). at the landscape scale (Loke et al. 2019c). Spatially heterogeneous mosaics also form in urbanized seascapes as a result of fine-scale gradients in nutrient enrich- Key ecological patterns ment and sediment pollution (Airoldi 2003, Baum et al. 2015, Ling et al. 2018), particularly in low flow environ- The convergence of exploitation, pollution and ocean sprawl ments and enclosed estuaries and embayments (Balls 1994, that typifies urban marine environments may lead to shifts in Dauer et al. 2000). For instance, physical disturbance from

1223 swing moorings, which are ubiquitous in shallow sedimen- (Terrados et al. 1998, Waycott et al. 2009, Polidoro et al. tary environments in Sydney Harbor, leads to depressed con- 2010, Heery et al. 2018a), loss in foundation species is gen- centrations of metal contaminants within a highly localized erally tied to one or more of the three major drivers of marine area (Hedge et al. 2017). This may result in complex, fine- urbanization (Rogers 1990, Hastings et al. 1995, Airoldi scale spatial patterns in microbial, meiofaunal and macrofau- 2003, Balestri et al. 2004, Kirby 2004, Connell et al. 2008, nal taxa that are sensitive to metal contamination (Coull and Strain et al. 2014, Yaakub et al. 2014a, Alleway and Connell Chandler 1992, Stark 1998, Lindegarth and Hoskin 2001, 2015). In temperate areas, nutrient-rich, high sedimentation Mucha et al. 2003, Gillan et al. 2005, Sun et al. 2012). It conditions can limit the recruitment and survival of canopy- is likely this is complicated further by localized gradients in forming kelps while supporting opportunistic, turf-forming other abiotic conditions, such as granularity, that commonly algal species that can act as kelp competitors (Airoldi 1998, occur in the vicinity of artificial structures (Martin et al. 2005, Benedetti-Cecchi et al. 2001, Gorgula and Connell 2004, Seitz et al. 2006). While swing moorings and other struc- Russell and Connell 2005, Coleman et al. 2008, Gorman tures that increase physical disturbance and scour increase and Connell 2009). Similarly, in the tropics, sediment pol- sediment grain size (Hedge et al. 2017), structures such as lution has multiple negative effects on corals. These decrease pilings that reduce flow speeds and increase deposition tend coral cover and disproportionately impact competitive, to reduce the grain size of nearby sediments (Heery et al. branching coral genera such as Acropora, which ultimately 2018b). Grain size, contaminant concentrations, and a vari- lowers reef complexity in urban areas (Heery et al. 2018a). ety of other flow-related metrics are known to have strong Ocean sprawl can also be an important driver of foundation effects on sedimentary composition and diversity (Mannino species loss. For instance, despite the numerous ecosystem and Montagna 1997, Hewitt et al. 2005), which likely varies services they provide to urban communities (Benzeev et al. considerably in urban seascapes over small spatial scales. 2017), mangrove forests are cleared in many coastal areas Studies of marine diversity and connectivity relative to to make way for urban development (Harper et al. 2007, urbanization remain relatively limited, and there is need for Martinuzzi et al. 2009, Lai et al. 2015, Richards and Friess expanded work in this area. In particular, study designs that 2016). Where urban mangroves are left intact, they are allow for the assessment of alpha, beta and gamma diver- vulnerable to deleterious effects from artificial structures con- sity could be helpful for beginning to distinguish between structed nearby; mangrove forests adjacent to seawalls tend to the ecological processes that shape marine assemblages in be narrower, with less leaf litter and fewer saplings than those spatially heterogeneous urban seascapes. In their eDNA without seawalls (Heatherington and Bishop 2012). Coral study on seagrass beds, Kelly et al. (2016) found decreases reefs and seagrass beds are also frequently built over (Chou in beta diversity even while species richness increased with 2006, Burt et al. 2013, Yaakub et al. 2014b). Furthermore, the intensity of urbanization. Landscape-scale homogeniza- urban losses in foundation species often involve feedbacks tion in urban assemblages has some precedents in freshwa- that prevent subsequent population recovery (Altieri and ter and terrestrial systems (McKinney and Lockwood 1999, Witman 2006, de Boer 2007, Moore et al. 2014). For Holway and Suarez 2006, Urban et al. 2006, Groffman et al. instance, seagrass loss can be tied to sediment pollution and 2017), but less so in the marine literature (Balata et al. 2007). eutrophication (Waycott et al. 2009, Orth et al. 2017) and For instance, by creating urban freshwater reservoirs/dams deforestation and altered hydrodynamic regimes from coastal many cities have inadvertently fragmented their catchments construction (da Silva et al. 2004), as well as possible indirect and resulted in biotic homogenization (Olden and Rooney effects from top to down reductions in grazers that control 2006, Olden et al. 2008). The straightening or ‘linearization’ seagrass epiphyte loads (Duffy et al. 2005, Myers et al. 2007). of shorelines through armoring (Dyl 2009) could homog- enize intertidal communities at certain scales, though this The reduction of seagrass bed cover can lead to destabilization has not been demonstrated empirically. Sedimentation may of sedimentary substrata, which then further increases tur- also cause marine communities to become more homogenous bidity (de Boer 2007) and potentially inhibits recolonization under certain conditions (Balata et al. 2007). However, more (Moore et al. 2014). thorough characterization of diversity measures relative to There is increasing evidence that multiple, often interact- resource exploitation, pollution and ocean sprawl should ing, urban-related drivers affect both foundation species and advance understanding of ecological processes in urban ecological response to foundation species loss (Lenihan and marine environments. Peterson 1998, Jackson 2008, Claudet and Fraschetti 2010, Nyström et al. 2012, Strain et al. 2014, Ferrario et al. 2016, Loss of foundation species Orth et al. 2017), although studies evaluating multiple urban stressors simultaneously are rare (O’Brien et al. 2019). The Urban stressors can be particularly detrimental for sensitive abundance of kelps and other important habitat-forming mac- foundation species such as oysters, reef-building corals, sea- roalgae is negatively correlated with human population den- grasses, mangroves and canopy-forming kelps, which structure sity in several regions, including temperate coasts in Australia marine ecosystems via the provisioning of biogenic habitat and North America (Connell et al. 2008, Scherner et al. (Dayton 1972, Bertness and Callaway 1994). Even though 2013, Feist and Levin 2016), and this is likely linked to gra- the dynamics of decline vary among taxa and across locations dients in sedimentation and nutrients (Fowles et al. 2018).

1224 Yet, ocean sprawl may also be an important factor in mac- as well as other factors (Alpine and Cloern 1992, Monbet roalgal community dynamics. Reduced topographic com- 1992). Nutrient loading therefore does not manifest complexity, changes in substrate type, and altered substrate parable, elevated marine production across cities. Moreover, profiles are all factors that can limit kelp abundance (Toohey broader ecosystem responses to primary production also vary 2007, Schroeter et al. 2015) and correlate with urban habi- across urban marine ecosystems. In some locations, nutri- tat conversion. Artificial structures not only support distinct ent enrichment can trigger micro- and macroalgal blooms macroalgal assemblages compared with natural rocky shores that are highly detrimental to important foundation species (Glasby 1999) – the kelps that inhabit them also support dis- (McGlathery 2001) while, in other places, the same process tinct epifaunal and microbial communities and erode at dif- may increase secondary production (Leslie et al. 2005) and ferent rates (Marzinelli et al. 2009, 2018, Mayer-Pinto et al. species richness (Whittaker and Heegaard 2003). 2018). Habitat conversion thus likely influences ecological processes in urban areas where canopy-forming kelps persist. Novel assemblages The interaction of resource extraction, pollution and ocean sprawl as drivers of foundation species loss, and the ecological Novel assemblage structure tends to emerge as species move responses to this loss, are important future areas of research. and change in abundance and dynamics in response to envi- Importantly, these processes are highly dynamic, with ecolog- ronmental change (Hobbs et al. 2018). The most obvious ical legacies from past impacts, and future scenarios linked to manifestation of this phenomenon in urban marine environments is among sessile assemblages on artificial shorelines. rising temperatures and pCO2, that are challenging to ascer- tain (Ramalho and Hobbs 2012, Davis et al. 2017, Gao et al. Conversion from natural shores to hard artificial struc- 2017, Heldt et al. 2018, Fig. 2). tures creates new habitats for colonization and supports novel assemblages of hard-bottom organisms (Chou and Changes in biodiversity and productivity Lim 1986, Connell and Glasby 1999, Bulleri et al. 2005, Moschella et al. 2005, Clynick et al. 2008, Lam et al. 2009, Patterns of biodiversity in urban marine environments are Airoldi et al. 2015, Munsch et al. 2015). These assemblages complex. Resource extraction, sediment pollution and habi- differ from nearby rocky shores with respect to composition tat modification are important drivers of marine biodiversity (Chapman 2003, Bulleri and Chapman 2010, Airoldi et al. declines globally (Sala and Knowlton 2006), and there are 2015, Lai et al. 2018) and genetic diversity (Fauvelot et al. many examples from the literature of reduced species rich- 2009). Differences in species abundance between artificial ness and altered community composition at heavily urban- and natural rocky shores may be biased towards some func- ized sites (Pearson and Rosenberg 1978, Long et al. 1995, tional groups, such as motile primary consumers (Chapman Lindegarth and Hoskin 2001, Lotze et al. 2006, Airoldi and 2003, Pister 2009). However, human-made habitats in urban Beck 2007, Poquita-Du 2019). Even though the diversity areas also provide a foothold for a variety of non-indigenous of marine assemblages in some regions is negatively corre- species, many of which are non-motile (Glasby et al. 2007, lated with human population density (Scherner et al. 2013, Vaselli et al. 2008, Ruiz et al. 2009, Sheehy and Vik 2010, Neo et al. 2017), this pattern is not universal, and varies con- Simkanin et al. 2012, Airoldi et al. 2015, Foster et al. 2016). siderably between regions, cities, the taxa and type of diversity considered, and the methods used. For instance, using Ruderal species and potential synanthropes eDNA from water samples, Kelly et al. (2016) found that species richness was positively correlated with land-based On land, urbanization is strongly associated with the pro- urbanization in intertidal seagrass beds. Similarly, while some liferation of ruderal and synanthropic species (McKinney studies have reported higher species diversity on artificial 2006). Ruderal species, those that grow in contaminated shorelines than on their natural counterparts (Chou and Lim soils or human wastes, typically include a variety of weedy 1986, Connell and Glasby 1999, Munsch et al. 2015), others plant species (Haigh 1980), while ‘synanthropes’ is a term have found artificial shorelines to be relatively depauperate typically applied to mid-level consumers, such as raccoons (Firth et al. 2013, Aguilera et al. 2014, Lai et al. 2018). and coyotes, that have higher densities and abundances in cit- There are similar complexities surrounding productivity in ies than in adjacent rural areas (McKinney 2002). Although urban marine environments. In nutrient-rich marine estuar- not well studied, there is evidence of analogue taxa exploit- ies, like those in most coastal cities, climate variables, such ing urban marine environments. Polluted sediments in urban as major precipitation events and interannual fluctuations in areas appear to generate opportunities for certain marine weather patterns, tend to be particularly important drivers of microbes (Córdova-Kreylos et al. 2006, Cetecioğlu et al. temporal patterns in primary production (Mallin et al. 1993, 2009, Nogueira et al. 2015). For instance, Alteromonadales, Rodrigues and Pardal 2015), as these events deliver land- Burkholderiales, Pseudomonadales, Rhodobacterales and based sources of nitrogen to coastal waters. However, the Rhodocyclales bacteria that are involved in the degradation relationship between nutrient load and primary production is of hydrocarbons, were found to be more abundant in pol- highly variable (Borum and Sand-Jensen 1996), and urban- luted urban mangrove forests in Brazil (Marcial Gomes et al. related increases in nutrient loads can have different effects 2008). Some macroalgae also respond opportunistically to depending on tidal regimes, the system’s trophic structure, polluted urban waters (Valiela et al. 1990, Raven and Taylor

1225 2003). For instance, transplant experiments have demon- they readily utilize artificial structures, such as floating docks, strated that the photosynthetic capacity of sea lettuce Ulva as habitat for settlement (Lambert and Lambert 1998, 2003, lactuca increases while that of canopy-forming brown sea- Piola and Johnston 2008, Dafforn et al. 2009, Piola et al. weed Sargassum stenophyllum decreases in response to urban 2009, Airoldi and Bulleri 2011, Edwards and Stachowicz waters (Scherner et al. 2012). Differential photosynthetic 2011, Gittenberger and van der Stelt 2011, McKenzie et al. responses to copper contaminants among different species of 2012, Simkanin et al. 2012, Cordell et al. 2013, Zhan et al. Ulva may connote a competitive advantage in contaminated 2015). In this way, simultaneous positive responses to mul- urban areas (Han et al. 2008). Similarly, the combination of tiple urban drivers may help to facilitate invasion success elevated sediment and nutrient loads increases the cover of in urban areas, although the strength of these responses filamentous turf-forming macroalgae in field manipulations likely vary between cities, taxonomic groups, and latitudes (Gorgula and Connell 2004) and is thought to be central (Canning-Clode et al. 2011). to turf proliferation in metropolitan areas (Airoldi 1998, Connell et al. 2008, Strain et al. 2014). Acclimatization and adaptation Evidence for synanthropic marine consumer species is more limited. Most of the studies on fish distribution patterns Urbanization is considered a major selective pressure in urban areas and relative to coastal population density sug- (Alberti 2015, Donihue and Lambert 2015) leading to phe- gest primarily negative impacts of urbanization on major fish notypic changes at both the organismal and species levels (Alberti et al. 2017a). These changes are either phenotypically groups (Toft et al. 2007, Williams et al. 2011, Kornis et al. plastic (i.e. within-lifetime) responses such as acclimatization, 2017, Munsch et al. 2017, Cinner et al. 2018). Although or (population-level) adaptation via genetic change over mul- several well-recognized terrestrial synanthropes, including tiple generations (Alberti et al. 2017b, Johnson and Munshi- raccoons and rats, are known to forage in intertidal habitats South 2017). Recent advances in understanding evolutionary (Carlton and Hodder 2003), degraded intertidal resources responses to urbanization have been driven largely by work in urban areas are unlikely to be a major driver of synan- in terrestrial systems (Partecke et al. 2006, Miranda et al. thropic distribution patterns for these species. There is at least 2013, Johnson and Munshi-South 2017). However, there is one record, however, of rats occurring in higher densities on ample precedent for rapid evolutionary change and pheno- artificial breakwaters than on natural shorelines (Aguilera typic plasticity in response to anthropogenic stressors in the 2018). Heery et al. (2018c) found that deep-dwelling giant marine environment (Todd 2008, Sanford and Kelly 2011). Pacific octopus were more common in urban than in rural All three of the key drivers of marine urbanization are areas of Puget Sound (northeast Pacific), and suggested this known to structure population genetics among a variety of may be a function of the amount of marine debris in the marine taxa (examples – resource exploitation: Smith et al. urban benthos, which octopus utilize as shelter (Katsanevakis 1991, Hauser et al. 2002; pollution: Suchanek 1993, and Verriopoulos 2004, Katsanevakis et al. 2007). Artificial López-Barea and Pueyo 1998, Nacci et al. 1999, Ma et al. structures, such as docks and buoys, are widely used as haul 2000, Virgilio et al. 2003, Virgilio and Abbiati 2004, out sites by pinnipeds (Heath and Perrin 2009) and could McMillan et al. 2006, Galletly et al. 2007, Moraga and similarly influence localized pinniped distribution patterns Tanguy 2009; ocean sprawl: Street and Montagna 1996, in urban areas (DeAngelis et al. 2008). Duarte et al. (2013) Fauvelot et al. 2012). In many cases, resource exploitation, noted that floating structures associated with coastal develop- pollution and ocean sprawl lead to population bottlenecks ment could play a key role in facilitating jellyfish blooms, by and reduced genetic diversity (Nevo et al. 1986, Maltagliati expanding the available habitat for polyp recruitment. These 2002, Fauvelot et al. 2009, Ungherese et al. 2010, Neo and lines of evidence suggest that, where synanthropic distribu- Todd 2012, Pinsky and Palumbi 2014). Yet evidence of tion patterns do exist among marine consumers, ocean sprawl micro-evolution in urban marine environments has been lim- may be an important underlying mechanism (Heery et al. ited. Some of the best examples come from the ecotoxicology 2018c). literature (Medina et al. 2007). For instance, McKenzie et al. In addition to ruderal macrophytes and synathropic con- (2011) showed heritable copper tolerance in the bryozoan sumers, the interacting drivers of marine urbanization appear Watersipora subtorquata. Similarly, Galletly et al. (2007) to facilitate the establishment of opportunistic sessile inver- found a significant geneotype × environment interaction in tebrates, many of which are non-indigenous. Opportunistic hatching success of the ascidian, Styela plicata, under dif- responses to multiple urban drivers may provide a particular ferent copper concentrations, yet hatching success at high advantage. For instance, the bryozoans, Bugula neritina and concentrations had a different genetic basis than that at low Watersipora subtorquata, and the ascidian, Botrylloides vio- concentrations, suggesting different genetic mechanisms for laceus, have particularly high tolerances for copper toxicity adaptation depending on pollution levels. (Piola and Johnston 2006), which could partially explain their Trait plasticity in response to marine urbanization has successful invasion of urban marine environments beyond been much more widely documented. Many marine organ- their endemic range (Piola et al. 2009, McKenzie et al. 2011, isms exhibit substantial capacity for acclimatization that Osborne et al. 2018). In addition, larval dispersal for these may provide a fitness advantage; this could include changes taxa is aided by shipping activities between coastal cities, and in morphology, physiology, behavior and/or life history

1226 (West-Eberhard 1989, Foo and Byrne 2016). Goiran et al. marine organisms that inhabit coastal defense structures (2017) observed melanism in sea snakes inhabiting urban (Ng et al. 2017), as well as for marine communities that pro- sites that they proposed facilitates the excretion of trace pol- vide sources of food and natural defenses for coastal cities, lutants. Phenotypically plastic responses to light in corals are such as coral reefs and mangrove forests (Ward et al. 2016, well documented and can benefit colonies where sediment Hoegh-Guldberg et al. 2017). Of course, coastal cities are pollution and associated turbidity is prevalent (Todd et al. also part of the problem as they contribute to climate change 2003, Hoogenboom et al. 2008, Ow and Todd 2010). Some via high levels of greenhouse gas emissions, energy consump- marine invertebrates also exhibit transgenerational plasticity, tion and changes in land use, hydrology and biodiversity wherein parents alter the phenotypes of gametes in response (Grimm et al. 2008a), but these additional impacts of marine to factors such as copper and salinity to maximize gamete per- urbanization are beyond the scope of the current review. formance (Marshall 2008, Jensen et al. 2014). Several other One of the better studied interactions between urbaniza- examples of trait plasticity from natural rocky shores may be tion and climate change is ‘coastal squeeze’, first reported additionally relevant in the abiotically stressful environments by Doody (2004), but later refined and defined by Pontee created by seawalls and other artificial structures (Strain et al. (2013, p. 206) as: ‘one form of coastal habitat loss, where 2018). For example, dog whelks Nucella lapillus and other intertidal habitat is lost due to the high water mark being gastropods have larger feet in high wave energy environments ﬁxed by a defence or structure (i.e. the high water mark resid- so they can adhere better to the substrate (Etter 1988, Trussell ing against a hard structure such as a sea wall) and the low 1997), potentially an advantage on steep seawalls that inten- water mark migrating landwards in response to SLR’ (sea sify wave shock. Similarly, local adaptation for thermal tol- level rise). Loss and/or fragmentation of tidal wetlands means erance in acorn barnacles Semibalanus balanoides (Bertness a concomitant reduction in ecosystem services, including and Gaines 1993) and acclimatization to high temperatures flood and erosion abatement, biodiversity support, water in various intertidal gastropods (Williams and Morritt 1995, quality, carbon sequestration and benefits to coastal fisher- Marshall et al. 2010) may facilitate survival in novel thermal ies (Torio and Chmura 2013). Managed retreat (or realign- environments associated with ocean sprawl. ment), where infrastructure is relocated inland to escape the Urbanization-driven trait changes can have important effects of erosion and flooding (Alexandrea et al. 2012), can effects on community interactions (Palkovacs et al. 2012, alleviate coastal squeeze by moving back or removing hard Alberti et al. 2017a), yet much work remains to understand artificial defences, thereby elimitaing the fixed high water the nature of these effects in the marine environment, as well as mark back-stop. However, the distances required for coasal their ultimate consequences for functioning in urban marine habitats to successfully move inland can be considerable – ecosystems. This work needs to be conducted across multi- potentially being meters per year depending on rate of sea ple organismal scales to account for potential urban-related level rise (Pethick 2001). acclimatization at the level of holobionts – host–microbial Temperature is a critical stressor on rocky shores (Helmuth assemblages that function as an ecological unit (Ziegler et al. and Hofmann 2001) but little is known regarding the ther- 2016, Evans et al. 2017). Further, the heritability of urban- mal landscape of artificial coast defenses (Zhao et al. 2019). driven adaptation should be considered through both genetic The homogeneity of artificial structures may create thermal and epigenetic approaches, as acclimatization responses can barrens that challenge intertidal organisms (Perkins et al. be inherited via transgenerational maternal effects and meth- 2015) or, alternatively, provide refugia from thermally-lim- ylation patterns (Sun et al. 2014, Suarez-Ulloa et al. 2015). ited predators. Helmuth et al. (2006), based on a compren- sive study of the spatial and temporal patterns in the body temperature of the mussel Mytilus californianus on natural Climate change and marine urbanization rocky shores, concluded that interacting factors such as tidal regime and wave splash can create complex thermal mosaics The effects of climate change interacting with marine urban- of temperature that are potentially more important locally ization range from reasonably established to complex and than those of large-scale (e.g. latitudinal) climate effects. speculative possibilities. Atmospheric warming from green- Hence, it will be difficult to predict or measure the broader house gases leads to the thermal expansion of the oceans and impacts of global warming on the intertidal area of seawalls melting of glacial and polar ice, and is well-documented as the and similar structures. Climate associated shifts in patterns cause of current and predicted sea-level rise (Neumann et al. of rainfall and runoff, e.g. heavier rainfall and/or more pro- 2015). Increases in the severity, and possibly occurrence, longed rainfall (Wallace et al. 2014), could overwhelm drain- of major storms have also been attributed to global warm- age systems leading to peaks in the influx of pollutants. These ing (Walsh et al. 2016). This combination of rising seas and unusual pollution spikes would likely be concurrent with extreme weather pose direct flooding and erosion threats to increased sedimentation, eutrophication and low salinity, all coastlines and, together with coastal development, represent of which could moderate species and community response the main drivers of the current proliferation of sea defenses and the toxicity of pollutants (Pearson and Rosenberg 1978, (Dafforn et al. 2015). Elevated temperatures, altered rainfall Šolić and Krstulović 1992, Verslycke et al. 2003). patterns, and other changes associated with climate change Climate change is also likely to impact natural coastal (Duffy et al. 2015, Donat et al. 2016) pose challenges for defenses. Healthy coral reefs and mangrove forests are effective

1227 at protecting coastlines from wave impact and associated ero- people and assets worth $13 trillion are going to be exposed sion in tropical and subtropical regions, but both are vul- to coastal hazards such as storms, flooding and climate vari- nerable to climate change. Extended periods of warmer than ability (Nicholls et al. 2013). Strategies that mitigate risk and average sea temperatures causes coral bleaching that, when help coastal cities adapt to sea level rise and climate change severe, kills colonies (Hoegh-Guldberg 1999) resulting in the are already being implemented in many parts of the world loss of wave-absorbing reef complexity (Alvarez-Filip et al. (Zimmerman and Faris 2010, Hayes et al. 2018) and are 2009, Graham and Nash 2013). As mangroves live within predicted to increase in the coming decades (Neumann et al. a narrow band of suitable habitat determined by local tidal 2015, Dangendorf et al. 2017). Such strategies, though mul- regimes, they are susceptible to sea level rise if it exceeds tifaceted, include expanded coastal armoring (French and the rate of soil accumulation, leading to shoreline retreat Spencer 2001, Hinkel et al. 2014), the integration of new (Lovelock et al. 2015). Many tropical and subtropical towns stormwater capture and treatment systems, and a wide vari- and cities benefit from the protection that coral reefs and ety of other modifications to increase the resilience of urban mangroves provide (Ferrario et al. 2014), and their loss can infrastructure (Zimmerman and Faris 2010). lead directly to the installation of alternative coastal defense If the past is any indication, future proliferation of marine measures, of which hard amour such as seawall, rip-rap and urbanization will further facilitate the formation of novel gabion are frequently chosen. There is also strong potential assemblages of marine organisms on an unprecedented scale. for additive or synergistic effects as coral reefs and mangroves Currently, there is considerable debate in ecology regard- near urban areas are likely to be heavily exploited as well as ing the concept of ‘novel ecosystems’ (Hobbs et al. 2014, impacted by pollution (Wells and Ravilious 2006). In addi- Murcia et al. 2014), i.e. ecosystems shaped by human inter- tion to these rather more predictable consequences of climate vention that are distinct from their historical state, and that change, urban marine environments – as part of urban eco- cannot be returned to their historical trajectory (Hallett et al. systems – are shaped by a multitude of interacting social and 2013). It is presently unclear whether urban marine ecosys- ecological drivers (Alberti et al. 2003) and are likely to exhibit tems meet all criteria of ‘novel ecosystems’ (Morse et al. 2014), non-linear dynamics characteristic of complex adaptive sys- but their trajectory is undeniably shaped by the way in which tems (Scheffer et al. 2001, Alberti 2008). The three major coastal cities develop and modify the marine environment drivers of marine urbanization have gradually altered urban (Dafforn et al. 2015). Given the potential of marine assem- marine ecosystems in ways that may have reduced their capac- blages to provide ecosystem services to urban populations, ity to absorb disturbance; for instance to a 100-year storm as well as recent success in the realm of eco-shoreline design event, a sudden change in socio-economic variables such as a (Toft et al. 2013, Morris et al. 2019), it may be more helpful rapid loss in food security, a major marine disease epidemic, to consider urban marine ecosystems and their future trajec- or various other pulse perturbations. Without considerably tory within the framework of ‘designed ecosystems’ (Higgs more research, it is unclear how urban marine ecosystems 2017) or ‘reconciliation ecology’ (Rosenzweig 2003a). While will respond to such disturbances, whether they are suscep- both of these frameworks arose with the realization that some tible to future phase shifts, and what such shifts might mean systems have been so severely altered and/or degraded it is for ecosystem functions and ecosystem services. While these practically impossible to apply conventional restoration prac- should be focal points of future research (discussed below), tices (or expect the system to shift back towards a ‘historic’ approaches such as scenario planning (Peterson et al. 2003) or ‘pre-disturbed’ state), conceptually they are fundamen- that integrate and accommodate uncertainties directly into tally different in their intent, starting point and develop- management of urban marine environments would be highly mental trajectory (Hunter and Gibbs 2007, Higgs 2017). beneficial (Alberti et al. 2003). For instance, ‘designed ecosystems’ often involve large-scale intervention efforts to create and sustain the system whereas ‘reconciliation ecology’ is less reliant on long-term interven- Ecological engineering tion and more based on the idea that ‘if you build it, they will come’ (Rosenzweig 2003b, p. 6). ‘Ecological engineering’, i.e. It is predicted that by the next decade approximately three the design and engineering of urban infrastructure congru- quarters of the world’s population will reside in coastal ent with ecological principles, can be viewed as straddling zones (Small and Nicholls 2003, Bulleri and Chapman between these frameworks, as it often requires huge initial 2015). Coastal land is therefore in high demand and devel- intervention but with less emphasis on subsequent man- opment and reclamation are occurring at unprecedented agement and maintenance (see recent review by Loke et al. scales (Yeung 2001, Duarte et al. 2008, Duan et al. 2016, 2019a). Chee et al. 2017, Sengupta et al. 2018). In addition, the Ecological engineering is currently being trialed, or risks of climate change, as outlined in the previous section, attempted in earnest, in many locations around the world have resulted in an urgent need for greater shoreline protec- (Chapman and Blockley 2009, Mitsch 2012, Strain et al. tion, especially in low-elevation coastal zones (LECZ) (sensu 2018). Nature-based or soft-engineering approaches using Neumann et al. 2015). For instance, in China, Japan and ‘green infrastructure’ for coastal defense are preferred over Korea alone, 28% of the global population are currently liv- hard engineering approaches in many coastal cities as they ing in LECZ and it is predicted that by 2070, 37 million have been shown to be more cost-effective in the longer term

1228 and can serve multiple functions in addition to flood risk 5. Assessment of the evidence for urban-driven trait selection reduction (Temmerman et al. 2013, Spalding et al. 2014, in the marine environment. Reguero et al. 2018). However, these solutions are often not 6. What ecological enhancement approaches (ecological adopted due to feasibility (e.g. mangrove planting at sites engineering, green- and blue-infrastructure, etc.) are most with high wave energy or flow) or socio-economic reasons effective in urban settings? (e.g. lack of political will, support or resources). In addition, There are also numerous questions related to the key ecolog- hard artificial coastal defenses have frequently already been ical processes discussed in the section ‘Key ecological patterns’ constructed and cannot realistically be removed. Given that that need to be elucidated, especially disentangling co-varying more human-made shorelines are expected to be built in stressors and determining the long-term responses of organ- the foreseeable future, it is critical to find ways to increase isms and populations to marine urbanization. Ultimately, all their ecological and social value while maintaining their aspects of coastal city design: architecture, urban planning engineering function (Borsje et al. 2011, Loke et al. 2019a). and civil and municipal engineering, will need to prioritize The ecological engineering of human-made shoreline struc- the marine environment if the negative effects of urbanization tures is a new but dynamic field, and there is often a tradeoff are to be minimized. In particular, planning strategies that between taking time to understand these habitats as a system, account for the interactive effects of drivers and accommo- and the urgency or desire to implement practical solutions date complex system dynamics should enhance the ecological (Morris et al. 2019). Knowledge of urban shoreline ecosys- and human functions of future urban marine ecosystems. tems and of strategies that effectively enhance ecosystem functioning and services should improve over time, as ecological enhancement and blue/green infrastructure projects Acknowledgements – We would like to thank Oikos for the become more common and are applied in a broader variety of opportunity to write this Forum article. Any opinions, findings urban marine environments (Pontee et al. 2016). Developing and conclusions or recommendations expressed in this material are and maintaining research collaborations with industry will be those of the authors and do not necessarily reflect the views of the essential to ensure that lessons from each of these projects are U.S. National Science Foundation. shared and translated into subsequent designs and engineer- Funding – This research is supported by the National Research ing solutions (Mayer-Pinto et al. 2017). Further, partnerships Foundation, Prime Minister’s Office, Singapore under its Marine with city governments and planners will be needed if eco- Science Research and Development Programme (award no. logical enhancement projects are to be applied concurrently MSRDP-05). This research was also supported with resources from the U.S. National Science Foundation Long-term Ecological with broader improvements in water quality and at a suffi- Research (LTER) Program (grant no. DEB-1027188 to CMS). cient scale to have long-standing benefits, and then carefully Author contributions – PAT and ECH contributed equally to this monitored over time. paper and are joint first authors.

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